1. Field of the Invention
The present invention relates to a method and an apparatus for manufacturing and testing for reliability devices having a coating of a thick film composition thereon and, in particular, to a method and an apparatus for detecting and eliminating entrapped gas bubbles from the coating of the thick film composition.
2. Description of the Prior Art
Because of their high volumetric efficiency and thus their small size multilayer capacitors are the most widely used form of ceramic capacitors for thick film hybrid microelectronic systems. These capacitors are fabricated by stacking and cofiring thin sheets of ceramic substrate on which an appropriate electrode pattern is printed. Each patterned layer is offset from the adjoining layers in such a manner that the electrode layers are exposed alternately at each edge of the assembly. The exposed edges of the electrode pattern are coated with a conductive thick film material to form what is known as the capacitor termination. The termination electrically connects all the layers of the structure, thus forming a group of parallel connected capacitors within the laminated structure. Capacitors of this type are frequently referred to as monolithic capacitors.
During the fabrication of these and other microelectric components that use coatings of thick film compositions it is known that air or other gas bubbles, hereafter collectively referred to as "gas bubbles", may become entrapped within the coating. In the case of the multilayer capacitor, for example, the presence of these gas bubbles may disturb the uniformity of the termination. Although the conductive continuity of the termination is not usually interrupted the high reliability required for most applications does not permit the continued existence of such defects. Thus, from a reliability point of view, the elimination of the entrapped gas bubbles is essential.
Since the formulations used for the terminations are necessarily opaque, containing as they do conducting metals, glass frits and other nontransparent components, it is very difficult to detect the presence of entrapped gas bubbles. In the case of rather large bubbles the coating may develop cracks during drying or firing over the position of the bubble. However, when the size of the gas bubble is small the only way to detect its presence is to dissect the finished device, after the drying and/or the firing thereof, at different positions along the device, in order to ascertain if the devices in a given batch have the entrapped gas bubbles which may lead to defects.
Accordingly, in view of the foregoing it is believed advantageous to provide a method and an apparatus for detecting the presence of entrapped gas bubbles in the coating of the thick film device and, if the bubbles are found to be present, to eliminate or significantly reduce the number of the gas bubbles that are found to be present.